Hydraulic drive systems of conventional construction machines (e.g., hydraulic excavators) include those controlling the delivery flow rate of the hydraulic pump (main pump) so that the delivery pressure of the hydraulic pump becomes higher than the maximum load pressure of a plurality of actuators by a target differential pressure. This control is called “load sensing control”. In such a hydraulic drive system performing the load sensing control, the differential pressure across each of a plurality of flow control valves is kept at a prescribed differential pressure by use of a pressure compensating valve so as to make it possible during the combined operation (operation of a plurality of actuators at the same time) to supply the hydraulic fluid according to a ratio corresponding to the opening areas of the flow control valves irrespective of the magnitude of the load pressure of each actuator.
Such a hydraulic drive system performing the load sensing control is described in JP,A 10-205501, for example. In the conventional technology, an unload valve is connected to a hydraulic fluid supply line to which the hydraulic fluid delivered from the main pump is led. The unload valve operates mainly in conditions in which the flow control valves are not operating (neutral state), limits the pressure in the hydraulic fluid supply line of the main pump (delivery pressure of the main pump) below a preset pressure of a main relief valve, and returns the delivery flow of the main pump to a tank in the neutral state. For this purpose, the unload valve is equipped with a spring for setting a target unload pressure and acting on the valve in the valve-closing direction. The delivery pressure of the main pump and the maximum load pressure are led to the unload valve to act on the valve in the valve-opening direction and in the valve-closing direction, respectively. The hydraulic drive system is configured to lead the tank pressure (approximately 0 MPa) to the unload valve as the maximum load pressure in the neutral state. With this configuration, when the delivery pressure of the main pump exceeds the target unload pressure (set by the spring) in the neutral state, the unload valve opens, returns the delivery flow of the main pump to the tank, and thereby controls the delivery pressure of the main pump to keep it within the target unload pressure.
Further, when an actuator is driven, due to the characteristics of the above-described configuration, the unload valve controls the delivery pressure of the main pump to keep it within the sum of the maximum load pressure and the target unload pressure by returning part of the delivery flow of the main pump to the tank when the differential pressure between the delivery pressure of the main pump and the maximum load pressure exceeds the target unload pressure set by the spring of the unload valve.